Organic electroluminescent devices (hereinafter also referred to as “organic EL devices”), which utilize electroluminescence (hereinafter abbreviated as “EL”) from organic materials, have been used as thin light-emitting materials. Organic EL devices are fully solid-state elements in the form of thin films and can emit light at a low voltage of several volts to several tens of volts. Organic EL devices have a variety of advantageous characteristics, such as high luminance, high luminescence efficiency, low profile, and lightweight.
For this reason, organic EL devices have been attracted attention as surface-emitting articles, such as backlights of various displays, display boards, e.g., billboards and emergency lights, and illumination sources. In particular, an organic EL device including a thin and lightweight resin substrate provided with a gas barrier layer has attracted attention as a light-emitting device because such an organic EL device has high flexibility (pliability) and can be naturally and elastically bent when disposed on curved members or bent as desired, and thus is beneficial for providing dramatic rendering and decoration.
To form a transparent conductive layer, e.g., an ITO (indium tin oxide) film, as a transparent electrode layer on a thin resin substrate, sputtering is often employed in view of its characteristics. Unfortunately, the formation of a transparent electrode layer by sputtering exposes a thin resin substrate to high temperature, and the resin substrate is thermally deformed, which impairs its smoothness and cause, for example, wrinkles. Consequently, cracks and wrecks are formed in the transparent conductive layer on the thin substrate, and such a transparent conductive layer is readily broken when it is bent.
To address this problem, a method of producing a flexible transparent conductive film and functional element has been disclosed. This method involves the formation of a transparent conductive layer with a fine conductive particulate oxide, such as a fine particulate ITO (indium tin oxide), on a 3 to 50 μm-thick resin substrate by a coating process, and then compression of the layer (see, for example, Patent Literature (PLT) 1).
In the case of forming a transparent conductive layer on a thin transparent resin substrate by a coating process, the thin transparent resin substrate is not thermally damaged during the formation. Unfortunately, it is revealed that the sheet resistance of the formed transparent conductive layer is far away from a desired resistance for an electrode of an organic electroluminescent device. To address this problem on the resistance, annealing can be performed at approximately 300° C. after the formation of a transparent conductive layer of ITO (indium tin oxide) to lower the resistance of the transparent conductive layer. Unfortunately, this annealing also causes thermal deformation of a thin transparent resin substrate like sputtering. Thus, it has been difficult to produce a transparent conductive article having a desired resistance using a thin resin substrate.
Meanwhile, an organic electroluminescent device including a transparent conductive layer formed with silver or an alloy primarily composed of silver is disclosed (see, for example, PLT 2). PLT 2 states that the formation of a transparent conductive layer with silver or an alloy primarily composed of silver achieves a thinner transparent conductive layer, i.e., a transparent conductive layer having highlight transmittance and sufficient conductivity, and the resulting organic electroluminescent device has enhanced luminescence efficiency and lifetime parameters.
Unfortunately, PLT 2 merely describes a method using an approximately 0.7 mm-thick glass substrate. PLT 2 has no description or suggestion on the use of a thin resin substrate having a thickness of 50 μm or less, thermal deterioration of the smoothness of such a thin resin substrate, importance of providing a transparent conductive layer formed on such a thin resin substrate with durability against bending stress, and ways to address the problems.
Another method of producing a transparent conductive film is also disclosed (see, for example, PLT 3), where the transparent conductive film includes a substrate, a modified polysilazane layer on the substrate, and a metal layer formed with silver or an alloy primarily composed of silver on the modified polysilazane layer. PLT 3 states that a method described therein can produce a transparent conductive layer having sufficient conductivity and light transmittance, as well as a high moisture-blocking effect.
In such a configuration, unfortunately, the formation of a metal layer composed of silver or an alloy primarily composed of silver directly on a modified polysilazane layer barely provides a uniform metal layer. Furthermore, PLT 3 has no specific description on the use of a thin resin substrate having a thickness of 50 μm or less, and no description or suggestion on bending durability of a transparent conductive layer on such a thin resin substrate and ways to address the problems.